Such alloys are used for die casting of for example automotive, transmission and engine parts. Therefore the alloy needs to have good mechanical properties also at elevated temperatures. Alloys for this use available on the market today include AS21, AS41 and AE42. The alloy AS21 has the following composition (Hydro Magnesium Specifications), 1.9-2.5 weight % Al, minimum 0.2 weight % Mn, 0.15-0.25 weight % Zn, 0.7-1.2 weight % Si, maximum 0.008 weight % Cu, maximum 0.001 weight % Ni, maximum 0.004 weight % Fe and maximum 0.01 weight % of other elements each. The alloy AS41B (ASTM B93-94a) contains 3.7-4.8 weight % Al, 0.35-0.6 weight % Mn, maximum 0.10 weight % Zn, maximum 0.60-1.4 weight % Si, maximum 0.015 weight % Cu, maximum 0.001 weight % Ni, maximum 0.0035 weight % Fe and maximum 0.01 weight % of other elements each. The alloy AE42 (Hydro Magnesium Specifications) contains 3.6-4.4 weight % Al, minimum 0.1 weight % Mn, maximum 0.20 weight % Zn, maximum 0.04 weight % Cu, maximum 0.001 weight % Ni, maximum 0,004 weight % Fe, 2.0-3.0 weight % RE and maximum 0.01 weight % of others each. RE refers to rare earth elements. All these alloys contain some iron and as iron is detrimental to the corrosion properties of magnesium aluminium alloys, manganese is used to control and reduce the iron content in the alloys.
In spite of this, the corrosion resistance of for example AS21 is not sufficient in e.g. automotive use. Car parts are subjected to a harsh environment especially at winter time when de-icing agents are applied to the roads. The alloy AE42 has good corrosion properties also in this environment, but it is more expensive than e.g. AS21. In addition, the casting properties are not as good as for the others, particularly due to a tendency to stick and solder to the die.
Alloys of this type are also described for example in Norwegian patent No. 121 753, U.S. Pat. No. 3,718,460 and French patent No. 1 555 251.
The object of the invention is to improve the corrosion resistance without detoriation of basic properties of magnesium-aluminium-silicon alloys. Another object is to avoid increased costs of the alloy.
These and other objects of the invention are obtained by the alloy as described below. The invention is further described and characterized by the accompanying patent claims.
The invention concerns a magnesium based alloy with improved corrosion resistance, containing 1.5-5 weight % Al, 0.6-1.4 weight % Si, 0.01-0.6 weight % Mn, 0.01-0.4 weight % RE. The content of impurities should be kept at a low level with maximum 0.008 weight % Cu, maximum 0.001 weight % Ni, maximum 0.004 weight % Fe and maximum 0.01 weight % of other elements each. Particularly, a Mn content of 0.05-0.2 weight % is favorable. In addition, it is preferable to add until 0.5 weight % Zn and especially 0.1-0.3 weight % Zn. This element has a positive effect on corrosion resistance. The rare earth elements used are preferably in the form of Misch metal. A preferred alloy contains 1.9-2.5 weight % Al, 0.7-1.2 weight % Si, 0.15-0.25 weight % Zn, 0.01-0.3 weight % RE and 0.01-0.2 weight % Mn. The invention also concerns a method of improving the corrosion resistance of magnesium, aluminium, silicon alloys where Mn is added in order to reduce Fe impurities, by keeping both Mn and Fe at a low level by adding small amounts of RE. It is preferred to keep the Mn content above 0.01 weight % and the RE content in the range 0.01-0.4.